cmsy10.tfm is indeed preloaded. The metric file is however read in again when the main font size is not 10pt because the font is used scaled 1095 and scaled 1200 respectively for 11pt and 12pt.
If you rename the modified cmsy10.tfm say to modcmsy10.tfm and do the same changes to cmsy5, cmsy6, cmsy7, cmsy8, cmsy9, cmbsy5, cmbsy6, cmbsy7, cmbsy8, cmbsy9 and cmbsy10, writing
\DeclareFontShape{OMS}{cmsy}{m}{n}{%
<5><6><7><8><9><10>gen*modcmsy%
<10.95><12><14.4><17.28><20.74><24.88>modcmsy10%
}{}
\DeclareFontShape{OMS}{cmsy}{b}{n}{%
<5><6><7><8><9>gen*modcmbsy%
<10><10.95><12><14.4><17.28><20.74><24.88>modcmbsy10%
}{}
in the preamble should bring you on track more easily.
You have also to teach pdftex that the fonts are really the old ones. So you should add to the preamble
\pdfmapline{+modcmsy10 CMSY10 <cmsy10.pfb}
\pdfmapline{+modcmsy5 CMSY5 <cmsy5.pfb}
\pdfmapline{+modcmsy6 CMSY6 <cmsy6.pfb}
\pdfmapline{+modcmsy7 CMSY7 <cmsy7.pfb}
\pdfmapline{+modcmsy8 CMSY8 <cmsy8.pfb}
\pdfmapline{+modcmsy9 CMSY9 <cmsy9.pfb}
\pdfmapline{+modcmbsy10 CMBSY10 <cmbsy10.pfb}
\pdfmapline{+modcmbsy5 CMBSY5 <cmbsy5.pfb}
\pdfmapline{+modcmbsy6 CMBSY6 <cmbsy6.pfb}
\pdfmapline{+modcmbsy7 CMBSY7 <cmbsy7.pfb}
\pdfmapline{+modcmbsy8 CMBSY8 <cmbsy8.pfb}
\pdfmapline{+modcmbsy9 CMBSY9 <cmbsy9.pfb}
You may also save a file modcmsy.map with the following contents
modcmsy10 CMSY10 <cmsy10.pfb
modcmsy5 CMSY5 <cmsy5.pfb
modcmsy6 CMSY6 <cmsy6.pfb
modcmsy7 CMSY7 <cmsy7.pfb
modcmsy8 CMSY8 <cmsy8.pfb
modcmsy9 CMSY9 <cmsy9.pfb
modcmbsy10 CMBSY10 <cmbsy10.pfb
modcmbsy5 CMBSY5 <cmbsy5.pfb
modcmbsy6 CMBSY6 <cmbsy6.pfb
modcmbsy7 CMBSY7 <cmbsy7.pfb
modcmbsy8 CMBSY8 <cmbsy8.pfb
modcmbsy9 CMBSY9 <cmbsy9.pfb
and call \pdfmapfile{+modcmsy.map}, which would have the same effect. The file can be in any place where pdftex looks for map files. For instance, with TeX Live on a GNU/Linux system it can be
~/texmf/fonts/map/modcmsy/modcmsy.map
Similarly, you can store the modified modcmsy10.tfm file as
~/texmf/fonts/tfm/modcmsy/modcmsy10.tfm
(and the others in the same directory). The \DeclareFontShape instruction can go, together with the \pdfmapfile line in a custom .sty package, say
~/texmf/tex/latex/modcmsy/modcmsy.sty
cmr10.tfmmight be included in the format file, so it isn't really looked for as a file. Tryfmtutil --all, but you have to make sure your changedcmr10.tfmis found then. Note that you're breaking your TeX distribution this way, be sure to unbreak it again later. – Stephan Lehmke Jun 14 '12 at 17:27fmtutils! I can't really dofmtutil --allsince I'm a user with restricted rights on a machine that is centrally administrated. – Hendrik Vogt Jun 14 '12 at 17:30fmtutilis the version for users (putting things in your home directory). Your admin will be usingfmtutil-sys. – Stephan Lehmke Jun 14 '12 at 17:42texmfs. And: how many (large) files will that produce, and how long will that take? – Hendrik Vogt Jun 14 '12 at 17:4512pt...) – Hendrik Vogt Jun 14 '12 at 18:00cmsy10.tfmis indeed preloaded. The metric file is however read in again when the main font size is not 10pt because the font is usedscaled 1095andscaled 1200respectively for 11pt and 12pt. Changing the name and modifying the connected\DeclareFontShapedeclaration seems the best road. – egreg Jun 14 '12 at 22:27